The synthesis and secretion of pulmonary surfactant, a complex mixture of lipids and proteins, by the alveolar eipthielium is essential for maintaining the structural integrity of the alveolus during respiration. Our previous study identified two nev hydrophobic proteins associated with pulmonary surfactant that conferred nearly complete surfactant-like properties to mixtures of phospholipids. One of these proteins, SPL(Phe), is synthesized by the alveolar Type II epithelial cell as a preproprotein of 40,000 Daltons. The present proposal is designed to characterize the post-translational events that convert the hydrophilic SPL(Phe) precursor to the extremely hydrophobic, biophysically active, mature peptide associated with surfactant phospholipids. These studies will characterize the intracellular forms and the kinetics of SPL(Phe) secretion in primary cultures of rat Type II epithelial cells. The proteolytic events resulting in processing of the secreted 42,000 Da SPL(Phe) proprotein to the mature 8,000 Da peptide will be discerned. The binding of processed SPL(Phe) to specific sites on the surface of the Type II cells and time course of internalization will also be characterized. Finally, the intracellular fate of SPL(Phe), internalized by receptor mediated endocytosis, will be studied to determine whether SPL(Phe) is degraded or routed to the lamellar body and co-secreted with surfactant phospholipids. These studies seek to characterize the synthesis and metabolism of an important pulmonary surfactant peptide. This peptide is an essential component of surfactant replacement preparations and confers surfactant-like properties to alveolar phospholipids. The design of an appropriate pharmacological approach for prevention-therapy of Respiratory Distress Syndrome requires a thorough knowledge of how this metabolic pathway is regulated and integrated with other components of the pulmonary surfactant system, particularly during the perinatal period.